Variable diameter rotor blade actuation system

ABSTRACT

A rotor blade actuation system is disclosed for controlling extension and retraction of an outboard blade section of a rotor blade by varying the speed and direction of rotation between a main rotor shaft and a blade actuation shaft. The rotor blade actuation system includes a drive gear on the main rotor shaft and a blade actuation drive gear on the blade actuation drive shaft. A primary drive unit is engaged with and transmits rotary motion from the main rotor shaft to the blade actuation shaft. The primary drive unit includes an input gear engaged with the drive gear. A primary drive is engaged with the drive gear through an input gear and a master clutch. An output drive is engaged with the blade actuation drive gear. A forward clutch controls engagement of a forward planetary gearset with the primary drive and the output drive. The engagement of the forward planetary gearset causes the blade actuation shaft to rotate in the same direction as the rotation of the main rotor shaft. A reverse clutch controls engagement of a reverse planetary gearset with the primary drive and the output drive. The engagement of the reverse planetary gearset with the primary drive and the output drive causes the blade actuation shaft to rotate in the opposite direction from the rotation of the main rotor shaft. A rotor blade actuation control unit controls engagement and disengagement of the forward and reverse clutches.

FIELD OF THE INVENTION

This invention relates to a Variable Diameter Rotor (VDR) system fortilt rotor and/or tilt wing aircraft, and more particularly, to a VDRactuation system for controlling extension and retraction of a pluralityof extendable rotor blades.

BACKGROUND OF THE INVENTION

A tilt rotor or tilt wing aircraft typically employs a pair of rotorsystems which are supported at the outermost end of a wing structure andare pivotable such that the rotors may assume a vertical or horizontalorientation. In a horizontal orientation (i.e., horizontal rotor plane),the aircraft is capable of hovering flight, while in a verticalorientation (i.e., vertical rotor plane), the aircraft is propelled inthe same manner as conventional propeller-driven fixed-wing aircraft.

Currently, tilt rotor and tilt wing aircraft employ conventionalfixed-diameter rotor systems which are aerodynamically andaeroelastically designed in a manner that attempts to blend thecompeting requirements for hovering and forward flight modes ofoperation. For example, with regard to hovering flight, it is generallyadvantageous to employ a large diameter rotor to improve hoveringperformance by lowering disk loading, reducing noise levels, andreducing downwash velocities. Conversely, a relatively small diameterrotor is desirable in forward flight to improve propulsive efficiency byminimizing blade aero-elastic properties, minimizing blade area, andreducing tip speed (Mach number).

Variable Diameter Rotor (VDR) systems are known to provide distinctadvantages over conventional fixed-diameter rotors insofar as suchsystems are capable of successfully operating in both modes ofoperation. That is, when the plane of the rotor is orientedhorizontally, the rotor diameter is enlarged for improved hoveringefficiency and, when oriented vertically, the rotor diameter is reducedfor improved propulsive efficiency.

An example of one VDR system and VDR blade assembly is shown in U.S.Pat. No. 3,768,923 which discloses a blade assembly with an outer bladesegment configured to telescope over a torque tube member. The size ofthe rotor diameter is varied by controlling the extension and/orretraction of the outer blade segment. The outer blade segment includesa structural spar which carries the primary loads of the outer bladesegment, a leading edge sheath assembly and trailing edge pocketassembly, which sheath and pocket assemblies envelop the spar section todefine the requisite aerodynamic blade contour. The torque tube memberis mounted to the rotor hub assembly. The spar member of the outer bladesegment slides over the torque tube member. In addition to supportingthe outer blade segment, the torque tube member functions to transferflapwise and edgewise bending loads to and from the rotor hub whileimparting pitch motion to the outer blade segment.

A retraction/extension mechanism is located within the torque tubemember and spar. In one embodiment of the invention, theretraction/extension mechanism includes a threaded jackscrew which maybe driven in either direction by a bevel gear arrangement disposedinternally of the rotor hub assembly. The jackscrew engages a pluralityof stacked nuts which are permitted to translate axially along thejackscrew upon rotation thereof. Centrifugal load straps extend fromeach nut and are affixed via a retention plate to the tip end of thespar member. As the jackscrew turns, the stacked nuts are caused totranslate inwardly or outwardly, thereby effecting axial translation ofthe outer blade segment. Systems relating to and/or further describingVDR systems are discussed in U.S. Pat. Nos. 3,884,594, 4,074,952,4,007,997, 5,253,979, and 5655,879.

U.S. Pat. No. 5,299,912 discloses another retraction/extension mechanismfor a variable diameter rotor system. The retraction/extension mechanismincludes coaxial rotor shafts which are engaged with the rotor blade.More particularly, the outer rotor shaft is attached to a rotor hubthrough a gimbaled bearing and provides rotational control over therotor blade. The inner rotor shaft is attached to a bevel gear which, inturn, meshes with a bevel pinion mounted on a jackscrew. The jackscrewis engaged with the outer blade segment as described above. Rotation ofthe inner shaft produces corresponding rotation of the jackscrew, whichextends or retracts the outer blade segment.

U.S. Pat. No. 5,299,912 also discloses a blade actuation system forcontrolling the extension and retraction of the outer blade segment bycontrolling the speed differential between the main rotor shaft and theblade actuation shaft.

The primary drawback of the prior art systems is that they were designedprimarily for developmental purposes. Those designs typically did notaccommodate the dynamic loads that are produced in full size hardwaredesigns, including gear tooth bending loads, and hertz stress toaccommodate the torque load required to operate a driving screw member.The driving screw torque is a direct function of the centrifugal forcecreated by the components of each blade assembly.

A need, therefore, exists for an improved blade actuation system for avariable diameter rotor system.

SUMMARY OF THE INVENTION

A rotor blade actuation system is disclosed for a variable diameterrotor system. The variable diameter rotor system includes a plurality ofrotor blade assemblies mounted to and rotated by a rotor hub assemblyabout an axis of rotation. Each rotor blade assembly has an inboardblade section and an outboard blade section. The outboard blade sectionis telescopically mounted to the inboard blade section. Actuation of theoutboard blade section is controlled by the rotor blade actuation systemby varying the speed and direction of rotation between a main rotorshaft and a blade actuation shaft.

The rotor blade actuation system includes a drive gear on the main rotorshaft which has a first set of teeth formed on it. A blade actuationdrive gear is formed on the blade actuation drive shaft and has a firstset of teeth on it.

A primary drive unit is engaged with and transmits rotary motion fromthe main rotor shaft to the blade actuation shaft. The primary driveunit includes an input gear engaged with the first set of teeth on thedrive gear. A primary drive is engaged with the input gear by means of amaster clutch. The master clutch controls transmission of rotary motionbetween the input gear and the primary drive.

An output gear is engaged with the first set of teeth on a bladeactuation drive gear. The output gear is also engaged with an outputdrive.

A forward planetary gearset is selectively engaged with the primarydrive and the output drive. The forward planetary gearset is operativefor transmitting rotary motion from the primary drive to the outputdrive. The engagement of the forward planetary gearset causes the bladeactuation shaft to rotate in the same direction as the rotation of themain rotor shaft.

A forward clutch controls the engagement of the forward planetarygearset with the primary drive and the output drive.

A reverse planetary gearset is selectively engaged with the primarydrive and the output drive. The reverse planetary gearset is operativefor transmitting rotary motion from the primary drive to the outputdrive. The engagement of the reverse planetary gearset with the primarydrive and the output drive causes the blade actuation shaft to rotate inthe opposite direction from the rotation of the main rotor shaft.

A reverse clutch controls the engagement of the reverse planetarygearset with the primary drive and the output drive.

The actuation control system also includes a rotor blade actuationcontrol unit for controlling engagement and disengagement of the forwardand reverse clutches.

In one embodiment of the invention, the rotor blade actuation controlunit includes a profile for controlling the engagement and disengagementof at least one of the clutches. The profile includes ramp-up andramp-down portions during which times the clutch is permitted to slip.

The primary drive unit may also include a brake which engages with theprimary drive. The rotor blade actuation control unit uses the profileto control engagement and disengagement of the brake.

In one embodiment of the invention, the rotor blade actuation systemfurther includes a secondary drive unit for transmitting rotary motionfrom the main rotor shaft to the blade actuation shaft in the event offailure of the primary drive unit.

The foregoing and other features and advantages of the present inventionwill become more apparent in light of the following detailed descriptionof the preferred embodiments thereof, as illustrated in the accompanyingfigures. As will be realized, the invention is capable of modificationsin various respects, all without departing from the invention.Accordingly, the drawings and the description are to be regarded asillustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, the drawings show a formof the invention which is presently preferred. However, it should beunderstood that this invention is not limited to the precisearrangements and instrumentalities shown in the drawings.

FIG. 1A is a plan view of a tilt-rotor aircraft illustrating thevariable diameter rotor system according to the present invention in itshorizontal position.

FIG. 1B is a front view of a tilt-rotor aircraft illustrating thevariable diameter rotor system according to the present invention in itsvertical position.

FIG. 2 is a cross-sectional side view of one embodiment of the variablediameter rotor system according to the present invention.

FIG. 3 is a cross-sectional top view of the variable diameter rotorsystem of FIG. 2.

FIG. 4 is a schematic representation of retraction/extension actuationsystem for controlling retraction and extension of a variable diameterrotor blade.

FIG. 5 is a graphical depiction of an engagement profile for use in theretraction/extension actuation system shown in FIG. 4.

FIG. 6A is a cross-sectional view of a linear constant velocity jointsuitable for use in the present invention.

FIG. 6B is a cross-sectional view of the linear constant velocity jointof FIG. 6A taken along lines 6B--6B.

FIG. 7 is a cross-sectional side view of an alternate rotor hub assemblyaccording to the present invention.

FIG. 8 is a cross-sectional view taken along lines 8--8 in FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numeralsillustrate corresponding or similar elements throughout the severalviews, FIGS. 1A and 1B illustrate a tilt rotor aircraft that includes apair of variable diameter rotor (VDR) systems 10 according to thepresent invention. The VDR systems 10 are shown pivotally mounted onlaterally extending wing sections 12 of the aircraft. The VDR systems 10are pivotable between a horizontal or hover flight position, shown inFIG. 1A, and a vertical or forward flight position, shown in FIG. 1B.

Each VDR system 10 includes a plurality of variable diameter rotor bladeassemblies 14 which are capable of being extended and retracted to varythe size of the rotor diameter RD as needed. More particularly, in orderto provide the maximum vertical lift for the aircraft, it is desirableto increase the size of the rotor diameter RD by extending the rotorblade assemblies 14. Conversely, in forward flight it is generally moredesirable to shorten the rotor diameter RD by retracting the rotor bladeassemblies 14.

In order to effectuate the change in diameter, the variable diameterrotor blade assemblies 14 include a torque tube member 16 and an outerblade segment 18 which telescopes over the torque tube member 16. Thegeneral structure of one suitable variable diameter rotor blade assembly14 for use in the present invention is disclosed in U.S. Pat. Nos.5,655,879 and 5,636,969, the disclosures of which are incorporatedherein by reference in their entirety.

The variable diameter rotor system 10 also includes a rotor hub assembly20 to which the rotor blade assemblies 14 are mounted. Referring to FIG.2, a cross-sectional view of the variable diameter rotor system 10 isshown illustrating the rotor hub assembly 20 in more detail. The rotorhub assembly 20 is mounted to a main rotor shaft 22 which is operativefor rotatably driving the rotor hub assembly 20 (and rotor bladeassemblies 14) about a rotational axis 24. The main rotor shaft 22 isconventional in the art and, thus, no further discussion is needed. Themain rotor shaft 22 is attached to a transmission (not shown) whichrotates the main rotor shaft in a prescribed direction and at a suitablespeed.

The main rotor shaft 22 is attached to the rotor blade assemblies 14through a gimbaled bearing 26 arrangement. The gimbaled bearing 26transmits rotor thrust loads to the main rotor shaft and permits the hubassembly 20 to have limited angular (pivotal) movement with respect tothe main rotor shaft 22. The gimbaled bearing 26 provides the ability totilt the rotor plane relative to the main rotor shaft 22 centerlineresulting in a thrust vector that is used to maneuver the aircraft inthe helicopter mode. The gimbaled pivot bearing 26 also provides reducedblade root stresses resulting from out of plane blade flapping motionthat occurs from rotor cyclic and vertical gust inputs. The gimbaledbearing 26 includes a lower bearing support 28 which is engaged with anupper portion of the main rotor shaft 22. Any conventional method forattaching the lower bearing support 28 to the main rotor shaft 22 can beused such that rotary motion can be transmitted therebetween. In onepreferred embodiment, the engagement between the lower bearing support28 and the main rotor shaft 22 is provided by a splined connection. Thelower bearing support 28 surrounds at least a portion of the upper endof the main rotor shaft 22 and includes a lower bearing surface 30 whichis preferably substantially hemispherical in shape.

A first elastomeric thrust bearing 32 is located adjacent to the lowerbearing surface 30. Elastomeric bearings are well known in the art andgenerally comprise alternating layers of elastomer and nonresilientshims. See, for example, U.S. Pat. No. 4,203,708. The number ofelastomer layers and shims is not limited to the number shown in thefigures but, instead, would be determined by the applied rotor loads.The first elastomeric bearing 32 is preferably hemispherical in shapeand located substantially concentric with the rotor hub assembly 20 soas to be rotatable about the rotational axis 24. A lower bearing member34 is disposed about and in contact with the outer surface of the firstelastomeric bearing 32, so as to sandwich the first elastomeric bearing32 between the lower bearing member 34 and the lower bearing surface 30.The lower bearing member 34 is attached to a rotor hub 39 fortransmitting rotational motion therebetween.

The gimbaled bearing 26 also includes an upper bearing support 36 whichis attached to the lower bearing support 28 through any conventionalmeans, such as a bolted. The upper bearing support 36 includes an upperbearing surface 38 which, in one embodiment, is preferably continuousabout the rotational axis 24 and has a center of rotation which issubstantially co-linear with the rotational axis 24. A secondelastomeric preload bearing 40 is disposed on the upper bearing surface38. The second elastomeric bearing 40 is constructed similar to thefirst elastomeric bearing 32. The second elastomeric bearing 40 ismounted so as to provide preloading of the bearing assembly 26. Thepreload is preferably equal to the rotor thrust load plus 1.5 g's. Thepreload is sized such that when the maximum thrust load is applied tothe first elastomeric bearing 32, no tensile stresses (i.e., completeloss of compression load) results in the second elastomeric bearing 40.

An upper bearing member 42 is disposed on the outer surface of thesecond elastomeric bearing 40. As such, the second elastomeric bearing40 is sandwiched between the upper bearing member 42 and the upperbearing surface 38. The upper bearing member 42 is attached to the hub39. A torque link 43 connects the upper bearing support 36 to the upperbearing member 42.

In the illustrated embodiment, the elastomeric spherical bearings 32, 40are not permitted to transmit torque since the application of torquewould reduce the angular capacity of the bearings. Instead, the presentinvention transmits drive torque around the spherical bearings by usinga link-type rotary coupling (constant velocity joint), similar to therotary coupling shown in U.S. Pat. No. 4,804,352, which is incorporatedherein by reference in its entirety. The rotary coupling 50 is shown ingreater detail in FIGS. 6A and 6B and will be discussed in more detailbelow. The elastomeric bearings 32, 40 are mounted so as to permitlimited pivoting of the rotor head assembly 20 about a pivot center 44which is located at the intersection of the rotational axis 24 and thelongitudinal pitch axis 46 of the rotor blade as shown in FIG. 3. Thatis, the elastomeric bearings 32, 40 are each preferably formed with ahemispherical shape and have a common center of curvature. The center ofcurvature defines the point about which the rotor hub assembly can pivotwith the least amount of out-of-plane loading (i.e., the pivot center).

The gimbaled arrangement shown in the figures and discussed abovepermits approximately 12° of pivotable motion. The 12° of motion allowsthe rotor head to vary the orientation of the rotor plane from itsnormal or true horizontal plane allowing thrust in the horizontal planewhen the aircraft is in the helicopter or vertical mode of flight.

In order to control extension and retraction of the outer blade segment18, the present invention includes a second drive shaft. As shown inFIG. 2, the second drive shaft or blade actuation shaft 48 is preferablylocated within the main rotor shaft 22. More particularly, the bladeactuation shaft 48 is preferably mounted concentrically within androtatable with respect to the main rotor shaft 22. The blade actuationshaft 48 is engaged with a suitable actuation system so as to berotatable relative to the main rotor shaft 22.

Since the rotor hub assembly is gimbaled, the attachment between theblade actuation shaft 48 and the rotor blade assemblies 14 must bedesigned to accommodate up to 12° of rotor hub pivoting, as well asvertical displacement caused by the thrust of the rotor. These thrustloads must be isolated from the blade actuation shaft 48. Theincorporation of a linear constant velocity joint 50 in the presentinvention at the elastomeric bearing pivot center provides the requiredisolation.

Referring to FIGS. 2, 6A and 6B, the linear constant velocity joint 50is used to couple the blade actuation shaft 48 to the rotor bladeassemblies 14. The constant velocity joint 50 is designed to transmittorque from the blade actuation shaft 48 to the rotor blade assemblies14, while permitting axial and pivotal motion of the rotor hub assembly20. In one embodiment of the invention, the constant velocity joint 50includes an output housing 52 and an input assembly 200. The inputassembly 200 is connected to the blade actuation shaft 48 through asplined connection 202. The input assembly 200 includes a tri-lobedtrunnion 204 with needle bearing rollers 206 rotatably mounted aroundeach trunnion 204. The outer race on the needle bearing rollers 206 isin rolling contact with the inner surface of the outer housing 52. Assuch, the coupling 50 transmits torsional input to the output housing 52while permitting low frictional axial (vertical) displacement of thehousing 52 with respect to the trunnion inner assembly 200.

As shown in FIG. 2, the housing 52 is preferably supported by a capmember 54 that is mounted to the hub 39. More particularly, at least oneroller bearing 56 is located between the housing 52 and a cylindricalflange 58 extending downward from the cap member 54. Thus, the housing52 is free to rotate with respect to the cap member 54.

A sun gear 60 is formed on a portion of the housing 52. While in theillustrated embodiment the sun gear 60 is shown as being integral withthe housing 52, it is also contemplated that the sun gear 60 can be aseparate gear that is attached to the housing 52. The sun gear 60 ispreferably made from a high strength material, such as steel.

In order to use the rotary motion of the sun gear 60 to extend andretract the outer blade segments 18, the present invention incorporatesa single stage planetary geartrain into the rotor hub assembly 20. Thesingle stage planetary provides two functions. First, the single stageplanetary reduces the actuation shaft 48 torque loads to a realisticoperating value. Second, the compound planetary provides a predeterminedtravel time for the blade extension and retraction motion. The bladetravel is a direct function of the actuation power source speed and therelated gearing associated with the drive mechanism and the systemactuation control unit.

Referring to FIGS. 2 and 3, the sun gear 60 provides input to the singlestage planetary geartrain. In one embodiment, the planetary geartrainincludes a pinion cage or support 70 which rotatably supports aplurality of pinion gears 62. The pinions gears 62 are disposed betweenand engage with the sun gear 60 and a ring gear 68. The ring gear 68 islocated radially outward from the pinion gears 62 and is attached to therotor hub 39 through any conventional means, such as a boltedattachment. The pinion gears 62 or the pinion cage 70 are interconnectedwith a hypoid bevel gear 76. More particularly, in the embodiment of theinvention shown in FIG. 2, the hypoid bevel gear 76 is fixedly attachedto the pinion cage 70 so as to be rotatable therewith about therotational axis 24. The hypoid bevel gear 76 interacts with an outputbevel pinion gear 78.

The intermeshing of the pinion gear 62 with the grounded ring gear 68results in the desired rotational motion of the pinion cage 70 and therelated hypoid bevel gear 76. The ring gear 68 and the pinion gears 62are preferably both made from steel. In one embodiment of the invention,the intermeshing of the ring gear 68 and the pinion gears 62 providesabout a 2.58:1 gear reduction and the single stage planetary gearsetproduces an overall gear ratio of 6:20:1. If desired, a compoundplanetary gear set may be configured to provide a gear ratio of 13:1 ifadditional reduction in applied torque and speed is required.

As shown in the illustrated embodiment, the pinion gears 62 are eachrotatably mounted to the pinion cage 70 through a roller bearing 72. Thepinion cage 70 is, in turn, in contact with and rotatably supported bythe hub 39 through a roller assembly 74. Any conventional rollerassembly 74 can be used to support the pinion cage 70. The lower hypoidbevel gear 76 is formed on the pinion cage 70 and defines a ring withinthe rotor hub assembly 20. The lower hypoid bevel gear 76 can beintegral with the pinion cage 70, or may be separately bolted to thesupport as is illustrated. The lower hypoid bevel gear 76 meshes withthe output pinion 78. As will be discussed in more detail below, theoutput pinion 78 is attached to the outer blade portion 18. The teeth onthe lower hypoid bevel gear 76 and the output pinion 78 are formed so asto result in an intermeshing between the two gears along a line thatintersects with the blade pitch pivot centerline 66 of the rotor blade18 as shown in FIG. 3. The blade pitch centerline 66 is offset from thepivot center 44. The output pinion 78 is located within the main rotorhub assembly 20 and, thus, allows the rotor hub to achieve upwards of 12degrees of pivotal motion with the gear mesh tracking with it. Theintermeshing between the lower hypoid bevel gear 76 and the outputpinion 78 defines a lower bevel stage of the planetary gear. The lowerhypoid bevel gear 76 and the output pinion 78 are both preferably madefrom steel. In one embodiment of the invention, the meshing between thetwo gears provides a gear reduction of about 3.58:1.

As shown, roller bearings 80 permit the output pinion 78 to freelyrotate about its rotational axis within the hub 39. The output pinion 78is attached to a drive shaft 82 preferably through a splined connection.The drive shaft 82 preferably includes crowned splines that are capableof accommodating misalignment resulting from motion of the jackscrew 84relative to its support bearing 86. In one embodiment of the invention,the quill shaft could include a disk type misalignment couplingidentified in FIG. 2 by the numeral 83. One suitable disk coupling 83 isa Lucas coupling sold by Lucas Co., of Utica, N.Y. The jackscrew 84 isrotatably mounted within the rotor hub 39 and torque tube member 16.

Since in-plane and out-of-plane motions occur on the rotor blade 18, aflexible elastomeric bearing support 86 is preferably incorporatedbetween the jackscrew 84 and the rotor hub 39 to reduce the effectivebending loads applied to the jackscrew 84. More particularly, ahemispherical bearing 87 is located between a complimentary screwsupport 88 and hub support 89. The screw support 88 is rotatablydisposed about the jackscrew 84. The hub support 89 is attached to thehub 39 through any conventional means known to those skilled in the art.The jackscrew 84 is preferably attached to the outer blade segment 18through any conventional means, such as the attachment disclosed in U.S.Pat. No. 5,636,969.

The above described design of the present invention, with the planetarygear set positioned above and to the sides of the gimbaled bearing 26,allows the entire rotor hub assembly 20 to be easily removed from themain rotor shaft 22, thus facilitating maintenance and repair.

In order to more clearly understand the extension and retractionmechanism of the present invention, a brief discussion of the rotorsystems' operation will now be provided. Starting with the variablediameter rotor blade assemblies 14 in their retracted position. The mainrotor shaft 22 is rotated as is conventional in the art (i.e., thetransmission rotates the main rotor shaft 22). For the illustratedaircraft configuration, a control mechanism is located in relation tothe main rotor shaft such that power to operate the extension/retractionmechanism is derived from the main rotor shaft 22 at 250 RPMs andresults in the rotation of the blade actuation shaft 48. This results inapproximately 500 foot-pounds of torque input to the blade actuationshaft 48. When commanded, the blade extension/retraction controlmechanism produces rotational forward and reverse output of the bladeactuation shaft 48 relative to the main rotor shaft 22 at approximately3144 RPMs. With the planetary gearset ratio of 6:20:1 and the hypoidgearset ratio of 3.58:1, the resulting jackscrew 84 rotational speed isapproximately 872 RPMs.

In the non-operating modes, i.e., fully extended or fully retracted, theblade actuation shaft 48 is not rotating relative to the main rotorshaft 22, but is rotating at the same speed (250 RPMs) as the main rotorshaft 22. The blade actuation shaft 48 rotates the housing 52 of theconstant velocity joint 50. This results in the sun gear 60 rotatingabout its rotational axis (which, when there is no deflection of therotor hub assembly 20, coincides with the rotational axis 24 of the mainrotor shaft 22 and the blade actuation shaft 48). The sun gear 60, inturn, rotates the pinion gears 62. Accordingly, the pinion gears 62simply rotate about their rotational axis 62A while, at the same time,transitioning around the rotor hub assembly 20 at the same speed as themain rotor shaft 22.

When it is desired to extend the outer blade segment 18 of the rotorblade assembly 14, the blade actuation shaft 48 must be controlled so asto rotate at a speed that is relative to the main rotor shaft 22. Thisrelative motion is either in the forward or reverse direction. Thegearing in one embodiment of the invention is sized such that therelative speed of the blade actuation shaft 48 with respect to the mainrotor shaft 22 in forward (extension) and reverse (retraction)directions is substantially the same. It must be noted that, since theretraction/extension mechanism is installed relative to the maintransmission housing, the output speeds are determined relative to the250 RPMs of the main rotor shaft 22 speed.

In the exemplary planetary gearset discussed above, the jackscrew 84rotates at about 872 RPMs and at approximately 350 foot-pounds islimited by the maximum centrifugal force in the blade extended position,the jackscrew lead, and the efficiency of the jackscrew nut for eachblade. This results in the outer blade segment 18 transitioning througha full 110 inches of extension in about 15 seconds.

Retraction of the outer blade segment 18 is accomplished by rotating theblade actuation shaft 48 again at a speed that is relative to the speedof the main rotor shaft 22 but in the opposite direction from extension.

FIG. 7 illustrates an alternate rotor hub assembly 400. Many features ofthis assembly 400 as the same as the prior assembly discussed in detailabove. As such, only the primary components that are different from theprior assembly are discussed hereinafter. The rotor hub assembly 400includes a gimbaled bearing arrangement 402 which attaches the mainrotor shaft 22 to the rotor blade assembles 14. As with the priorarrangement, the gimbaled bearing 402 transmits rotor thrust loads tothe main rotor shaft 22 while permitting the hub assembly 400 to havelimited angular movement.

The gimbaled bearing 402 includes a lower bearing support 404 which isattached to the main rotor shaft 22. A first elastomeric preload bearing406 is located between the lower bearing support 404 and a lower surfaceof a blade bearing mount 408. A second elastomeric thrust bearing 410 islocated between an upper surface on the blade bearing mount 408 and anupper bearing support 412. The upper bearing support is attached to themain rotor shaft 22 though any conventional mounting arrangement, suchas the illustrated splined connection. The blade bearing mount 408 isfixedly attached to the rotor blade hub 39. A torque link 409 connectsthe blade bearing mount 408 to the upper bearing support 412 as shown inFIG. 8.

One common misconception in the prior art was that in order to provideproper pivoting of the rotor hub, the gimbaled bearings had to bearranged with their pivot center located on the center line of the rotorblade assembly. The primary deficiency with this mounting arrangement isthat the blade typically has a pre-cone. As such, the center of mass ofthe blade does not typically lie along the center line of the rotorblade. Accordingly, the blade in these prior art arrangements would tendto droop when the aircraft was not in use, resulting in undesirableloading on the bearing from out-of-plane runout during start-up of therotor. In one embodiment of the invention, this problem is alleviated bymounting the elastomeric bearings 32, 40 with their pivot center locatedat the intersection of the rotational axis 24 and a horizontal planepassing through the mass center of the rotor blade assembly 14. Sinceconventional rotor assemblies, such as the rotor assembly shown in FIG.7, are designed with a "pre-cone", the mass center of the rotor bladeassembly 14 will be located above the point where the pitch axiscenterline 46 intersects the rotational axis 24. For example, in therotor hub assembly 20 shown in FIG. 7, the pivot center 44 is locatedapproximately 4.5 inches above the intersection of the blade pitch axis46 with the rotational axis 24 (identified as D).

As discussed previously, the blade actuation shaft 48 is engaged to aplanetary gearset through a liner constant velocity joint 50. As shownin this alternate embodiment, the outer housing 52 of the constantvelocity joint 50 has a sun gear 414 formed on it. The sun gear 414meshes with a pinion gear 416 that is rotatably supported by a pinioncage 418. The pinion cage 418, in turn, is rotatably supported by theouter housing 52 through a roller arrangement. The pinion gear 416 alsomeshes with a movable ring gear 420. The movable ring gear is disposedabout the pinion cage 418 and is rotatably engaged with the rotor hub39.

A lower hypoid pinion gear 76 is attached to or formed on the movablering gear 420 and drivingly engages with the output pinion 78 forrotating the jackscrew 84 as discussed in the prior arrangement.

A fixed ring gear 422 is attached to the rotor hub 39 and meshes with asecondary pinion 424 that is splined to the pinion gear 416 so as to berotatable in combination therewith.

The operation of the geartrain shown in FIGS. 7 and 8 will now bebriefly discussed. The illustrated geartrain is a compound planetaryversion which allows for higher reduction ratios than can be achievedwith a basic single stage. The compound planetary in this embodimentutilizes a double pinion arrangement and two ring gears. The input fromthe sun gear 414 rotates the first stage pinion gear 416 that isinterconnected with a second stage pinion gear 424. The multiple pinionslocated on the pinion housing/cage 418 rotate around the input sun gear414. The output is taken from the first stage ring gear 420 which isdriven by the first stage pinions 416. The ring gear 420 is mechanicallyattached to the hypoid bevel gear 76. The rotational output of theplanetary system is the result of the interaction of the second stagepinion 424 through the second stage ring gear 422 that is grounded tothe rotor housing 39. In one embodiment of the invention, the planetarygeartrain provides a reduction ratio of approximately 13:1.

The rotational sense of the planetary system is such that for eitherblade extension or retraction, all of the basic components rotate in thesame direction. As in the description of the previously describedembodiment the actuation is derived from the rotation of the main rotorshaft which rotates at 250 RPMs. An input rotation of the main rotorshaft 22 and the interconnected sun gear 414 causes a rotation of thedouble pinion that is opposite in sense. The cage housing 418 howeverrotates in the same sense as the sun gear 414. This rotational speed anddirection is determined by the relative size of the gears in theplanetary set. The relationship of the larger first stage pinion 416 andthe smaller second stage pinion gear 424, that is reacting with thesecond stage ring gear 420, results in a rotational sense of the ringgear 420 that is in the same direction as the input sun gear 414, but ata speed relative to the reduction ratio speed of the planetary. Rotationof the ring gear 420 produces rotation of the output pinion 78.

In order to reduce the loading on the extension/retraction mechanismand, in particular, on the straps used to attach the outer blade segment18 to the torque tube member 16, the present invention contemplates theincorporation of a locking mechanism 90 into the rotor blade assembly14. The locking mechanism preferably locks the outer blade segment 18 tothe torque tube and/or rotor hub 39 when the outer blade segment 18 isfully retracted. This eliminates the centrifugal loading that wouldotherwise be on the extension/retraction mechanism. In one embodiment ofthe invention, the locking mechanism 90 is a pin or breech type lockingdevice. More particularly, upper and lower locking pins 92 are mountedto corresponding actuators 94 which, in turn, are supported by flangeson the hub or blade root. The locking pins 92 are mounted so as toengage with holes (not shown) formed in the outer blade segment 18 whenthe outer blade segment 18 is in its fully retracted position.

The benefits obtained from the present invention over prior art systemsinclude the ability to provide controlled rates of the retraction orextension times for the outer blade segment 18 at a predetermined bladeactuation shaft maximum input torque. The invention also provides ameans for transmitting the operating torque through the angulardisplacement of a gimbaled rotor head while accommodating the axialdisplacement in the elastomeric gimbal bearing due to thrust loads.

Referring now to FIG. 4, a schematic diagram is shown depicting anembodiment of a preferred retraction/extension actuation system 100 forcontrolling the rotational speed of the blade actuation shaft 48 and,thus, the extension and retraction of the outer blade segment 18. Theretraction/extension actuation system 100 is preferably mounted at ornear the lower ends of the main rotor shaft 22 and the blade actuationshaft 48. The retraction/extension actuation system 100 providesapproximately 300 HP (i. e., 100 HP per rotor blade) to controlextension and retraction of the rotor system 10.

The retraction/extension actuation system 100 includes a bevel drivegear 102 which is shown disposed about and splined to the main rotorshaft 22 so as to be rotatable in combination therewith. The bevel drivegear 102 has a set of primary teeth 104 which mesh with mating teeth onan input gear 106 in a primary retraction/extension drive unit 108. Theinput gear 106 is engaged with a first or master clutch 110 whichcontrols the engagement between the input gear 106 and a main drive 112.The main drive 112, in turn, is engaged with a fixed mount 114 via adisk brake 116. The brake 116 is actuated during blade extension as willbe discussed in more detail below. The main drive 112 engages a forwardfixed sun gear planetary gearset 124 and with a reverse fixed pinionplanetary gearset 118.

A forward clutch 122 controls engagement between the main drive 112 anda forward planetary gearset 124. The forward planetary gearset 124 isdefined as a fixed sun, single stage planetary which includes multiplepinions 125, an outer ring gear 126, and an inner sun gear 127. Theforward clutch 122 is interconnected to the cage of the multiple pinions125. The sun gear is interconnected to ground 120 through a cage ofmultiple pinions 121 on the reverse fixed pinion gearset 118. When theforward clutch 122 is engaged, the pinions 125 of the forward planetarygearset 124 rotate about the sun gear 127 resulting in rotation of theouter ring gear 126 in the forward direction. The outer ring gear 126 isinterconnected through an output shaft 128 to output pinion 132.

The reverse planetary 118 is defined as a fixed pinion, single stageplanetary which includes multiple pinions 121, an outer ring gear 129,and an inner sun gear 123. The input of a reverse clutch 130 isinterconnected to the sun gear 123 and the output is interconnected tothe output shaft 128. When the reverse clutch 130 is engaged, rotationalinput from the main drive 112 rotates the outer ring gear 129. The fixedor grounded cage of pinions 121 causes the sun gear 123 to rotate in theopposite direction, thereby resulting in reverse rotation of the outputshaft 128.

The output drive 128 is coupled to an output bevel gear 132 which, inturn, is engaged with a primary set of teeth 133 on a blade actuationdrive gear 134 formed on the blade actuation shaft 48.

A rotor blade actuation control unit 136 controls the engagement betweenthe various clutches and the brake 116 in the primaryretraction/extension drive unit 108. In one preferred embodiment of theinvention, the actuation control unit 136 controls the actuation cycleby regulating engagement of one or more of the clutches and brake 116according to a profile. More particularly, referring to FIG. 5, apreprogramed profile is shown for controlling actuation of theextension/retraction drive unit 108. The profile includes a ramp-up inoperational speed that lasts for approximately 2-3 seconds. Moreparticularly, during the ramp-up, the engagement between the masterclutch 110 and the main drive 112 is allowed to slip until fullyengaged. Similarly, there is a ramp down in operational speed when it isdesired to disengage the clutches. The ramp-down also lasts forapproximately 2-3 seconds, during which time the master clutch 110 ispermitted to slip while operating in conjunction with brake 116. Thiscooperation of the master clutch 110 and brake 116 occurs during a bladeextend cycle operation. During a blade extend cycle, the forward clutch122 and the master clutch 110 are engaged. At the point of ramp-down,the forward clutch 122 is disengaged and modulated application of thebrake 116 occurs with modulated disengagement of the master clutch 110to ease the outer blade segment into its completely extended position.The engagement profile with its ramp-up and ramp-down portions helps toreduce the high load spike that would otherwise result in the system.

It is also contemplated that the rotor blade control unit 136 willmonitor the blade operating system condition parameters to determine theoverall health status of the rotor system. For example, the rotor bladecontrol unit 136 may receive signals representing blade position,actuating oil pressure, and oil temperature. From this information a"go/no-go" decision can be made by the control unit 136. Additionally,the monitored information can be used to adjust the profile if needed.For example, the actual blade position can be monitored to adjust whenthe ramp-up and ramp-down occurs.

It is also contemplated that a secondary retraction/extension drive unit140 may be incorporated into the system to provide redundancy in case ofa failure in the primary drive unit. It is further contemplated that thesecondary retraction/extension drive unit 140 can be designed to engagewith a set of secondary teeth 142 located adjacent to the drive gear102. Similarly, the output bevel gear in the secondaryretraction/extension drive unit 140 can be configured to engage withsecondary set of teeth 144 on a blade actuation shaft drive gear 134.The use of primary and secondary sets of teeth on the drive gear 102 andthe blade actuation drive gear 134 is intended to provide redundancy inthe system. If redundancy is not a concern, only one set of teeth oneach gear would be required.

As shown in FIG. 4, the gearing arrangement in the secondaryretraction/extension drive unit 140 is substantially the same as thegearing arrangement in the primary retraction/extension drive unit 108.Hence, no further discussion is needed. As with the primaryretraction/extension drive unit 108, the secondary retraction/extensiondrive unit 140 is controlled by the rotor blade retraction/extensioncontrol unit 136.

The retraction/extension actuation system 100 is preferably arrangedwith the clutches oriented vertically as shown. This alleviates anypressure differentials and/or gyroscopic loads that might otherwisedevelop if the unit incorporated horizontally oriented clutches. Theclutches and brakes that are used in the present invention arepreferably an oil type that will have nearly identical static anddynamic coefficients of friction. In addition to the beneficialcoefficient of friction, oil or wet type clutches and brakes also havebetter improved lubrication, and system health monitoring capabilityover dry type devices.

It is desirable that a gear ratio of 2:1 exist between the drive gear102 and the input gear 106. It is also desirable that a gear ratio of2:2 exist between the output bevel gear 132 and the blade actuationshaft 48.

The retraction/extension actuation system 100 also preferably includes abrake unit 146 which is used to lock the main rotor shaft 22 to theblade actuation shaft 48. The rotor blade control unit 136 preferablycontrols engagement of the brake unit 146. By allowing the rotor shaftsto be locked into engagement, it is possible to unload (disengage) theprimary drive unit 108 or secondary drive unit 140. The application ofthis function is desirable in the event of a detected malfunction ineither the primary drive unit 108 or the secondary drive unit 140 duringoperation, allowing the system to lock in a fixed portion. Brakingsystems are well known in the art. See, for example, U.S. Pat. No.5,299,912, the disclosure of which is incorporated herein by referencein its entirety.

The operation of the retraction/extension actuation system 100 will nowbe discussed. When the variable diameter rotor blade assembly 14 is inits retracted position (i.e., with the outer blade segment fullyretracted), the main rotor shaft 22 and the blade actuation shaft 48will be rotating concurrently at the same speed and in the samedirection. The input gear 106 is in constant engagement with the mainrotor shaft bevel gear 102 through the bevel gear mesh 104. Consequentlythe input gear 106 rotates at a constant 500 RPMs.

To initiate an extension or a retraction operation, the rotor bladeretraction/extension control unit 136 commands engagement of theappropriate clutch (e.g., forward clutch 122) in the drive unit 108 andthen the master clutch 110. The rotor blade retraction/extension controlunit 136 sequences the engagement of the master clutch 110 and brake 116to achieve the desired profile as shown in FIG. 5. At the completion ofthe retraction or extension operation, the drive unit 108 is disengaged.

When the blade is fully retracted or extended, the blade extensionmechanism must be restrained or locked so that the blade actuation shaft48 can once again rotate at the same speed and in the same direction asthe main rotor shaft 22. There are two methods for achieving this in theillustrated embodiment. First, the brake 146 can be applied to lock theshafts together. However, a more preferred method is to use the bladelocking mechanism 90 to lock the outer blade segment. As discussed aboveand shown in FIG. 2, the blade locking mechanism 90 locks the outerblade segment to the blade root or hub 39 when the blade is in itsretracted position. In order to unload the blade extension mechanism 100when the blades are in the extended position, internal stops (not shown)are incorporated in the blades. The outer blade segment 18 contacts thestops when it reaches the fully extended position and the rotor blade'scentrifugal force maintains blade in the extended position. Theunloading of the blade extension/retraction mechanism 100 minimizesfatigue loading.

In order to initiate a blade extension from the retracted position withthe blade locking mechanism 90, the centrifugal load must first berelieved in order to disengage the locking device. To accomplish this,the rotor blade/extension control unit 136 first initiates a bladeretraction operation to "pull-in" the blades to relieve the centrifugalload on the blade locking mechanisms 90. This retraction sequenceutilizes the same clutches and the brake used for the normal retractionoperation, however the cycle will not follow the operation profile. Moreparticularly, first the reverse (retract) clutch 130 is engaged,followed by the master clutch 110 to relieve the blade centrifugal load.At this point, the position brake 146 is engaged and the lockingmechanisms 90 are disengaged (as determined by sensors). The forward(extend) clutch 122 is then engaged with the subsequent engagement ofthe master clutch 110 and the concurrent controlled release of theposition brake 146.

The outer blade segments 18 extend in accordance with the normaloperation profile shown in FIG. 5. At the 2-3 second ramp-down point,the application of brake 116 is controlled with the concurrent releaseof the master clutch 110, easing the blade onto the position stops.

When it is desired to retract the outer blade segment 18, the rotorblade control unit 136 initiates the following sequence of events. Thereverse (retract) clutch 130 is engaged and then a controlled slipengagement of the master clutch 110 occurs to meet the 2-3 second"ramp-up" profile as defined in FIG. 5. As the outer blade segments 18approach the retracted stop position, the rotor blade control unit 136commands a controlled slip of the master clutch 110 to meet the"rampdown" profile specified in FIG. 5. At the point of engagement withretract position stops, the brake 146 engages and the blade lockingmechanisms 90 are engaged. After the lock position sensors signalcompletion of the lock engagement sequence, the brake 146 is controlledto ease the centrifugal load onto the locks.

The above retraction/extension system 100 provides a novel mechanism forcontrolling the extension and retraction of a variable diameter rotorblade, such as the exemplary embodiment provided in this specification.The choice of gear set ratios in the retraction/extension system 100 maybe selected to accommodate various rates of retraction or extension.

Although the invention has been described and illustrated with respectto the exemplary embodiments thereof, it should be understood by thoseskilled in the art that the foregoing and various other changes,omissions and additions may be made therein and thereto, without partingfrom the spirit and scope of the present invention.

What is claimed is:
 1. A rotor blade actuation system for a variablediameter rotor system, the variable diameter rotor system including aplurality of rotor blade assemblies mounted to and rotated by a rotorhub assembly about an axis of rotation, each rotor blade assembly havingan inboard blade section and an outboard blade section, the outboardblade section being telescopically mounted to the inboard blade section,actuation of the outboard blade section is controlled by the rotor bladeactuation system by varying the speed and direction between a main rotorshaft and a blade actuation shaft, the rotor blade actuation systemcomprising:a drive gear on the main rotor shaft and having a first setof teeth thereon; a blade actuation drive gear on the blade actuationdrive shaft and having a first set of teeth thereon; a primary driveunit for transmitting rotary motion from the main rotor shaft to theblade actuation shaft, the primary drive unit includingan input gearengaged with the first set of teeth on the drive gear, a primary drive,a master clutch disposed between the input gear and the primary drivefor controlling transmission of rotary motion therebetween, an outputgear engaged with the first set of teeth on a blade actuation drivegear, an output drive engaged with the output gear, a forward planetarygearset operative for transmitting rotary motion from the primary driveto the output drive, a forward clutch for engaging the forward planetarygearset with the primary drive and the output drive, the engagement ofthe forward planetary gearset causing the blade actuation shaft torotate in the same direction as the rotation of the main rotor shaft, areverse planetary gearset operative for transmitting rotary motion fromthe primary drive to the output drive, and a reverse clutch for engagingthe reverse planetary gearset with the primary drive and the outputdrive, the engagement of the reverse planetary gearset causing the bladeactuation shaft to rotate in the opposite direction from the rotation ofthe main rotor shaft; and a rotor blade actuation control unit forcontrolling engagement and disengagement of the forward and reverseclutches.
 2. A rotor blade actuation system according to claim 1 whereinthe rotor blade actuation control unit includes a profile forcontrolling the engagement and disengagement of at least one of theclutches.
 3. A rotor blade actuation system according to claim 2 whereinthe profile includes ramp-up and ramp-down portions during which timesthe at least one clutch is permitted to slip.
 4. A rotor blade actuationsystem according to claim 2 wherein the primary drive unit furtherincludes a brake which is adapted to engage with the primary drive, andwherein the rotor blade actuation control unit uses the profile tocontrol engagement and disengagement of the brake.
 5. A rotor bladeactuation system according to claim 4 wherein the rotor blade actuationcontrol unit utilizes the profile to modulate disengagement of the brakewhile concomitantly modulating engagement of the master clutch.
 6. Arotor blade actuation system according to claim 1 wherein the gear ratiobetween the drive gear and the input gear is about 2:1.
 7. A rotor bladeactuation system according to claim 1 wherein the gear ratio between theoutput gear and the blade actuation drive gear is about 2:2.
 8. A rotorblade actuation system according to claim 1 further comprising a brakeunit adapted to engage the main rotor shaft and the blade actuationshaft for locking the blade actuation shaft to the main rotor shaft. 9.A rotor blade actuation system according to claim 1 further comprising asecondary drive unit for transmitting rotary motion from the main rotorshaft to the blade actuation shaft in the event of failure of theprimary drive unit, the secondary drive unit includinga secondary inputgear engaged with a second set of teeth on the drive gear, a secondaryoutput gear engaged with a second set of teeth on a blade actuationdrive gear, a secondary drive, an secondary output drive engaged withthe secondary output gear, a secondary master clutch disposed betweenthe secondary input gear and the secondary drive for controllingtransmission of rotary motion therebetween, a secondary forwardplanetary gearset operative for transmitting rotary motion from thesecondary drive to the secondary output drive, a secondary forwardclutch for engaging the secondary forward planetary gearset with thesecondary drive and the secondary output drive, the engagement of thesecondary forward planetary gearset causing the blade actuation shaft torotate in the same direction as the rotation of the main rotor shaft, asecondary reverse planetary gearset operative for transmitting rotarymotion from the secondary drive to the secondary output drive, and asecondary reverse clutch for engaging the secondary reverse planetarygearset with the secondary drive and the secondary output drive, theengagement of the secondary reverse planetary gearset causing the bladeactuation shaft to rotate in the opposite direction from the rotation ofthe main rotor shaft; andwherein the rotor blade control unit controlsactuation of the clutches in the secondary drive unit.
 10. A variablediameter rotor system for an aircraft comprising:a main rotor shafthaving an upper end, a lower end and an axis of rotation; a plurality ofrotor blade assemblies, each rotor blade assembly having an inboardblade section and an outboard blade section, the outboard blade sectionbeing telescopically mounted to the inboard blade section; a screw driverotatably disposed within the rotor blade assembly and engaged with theoutboard blade section for telescoping the outboard blade section withrespect to the inboard blade section, the jackscrew having an outputpinion located at its inboard end, a rotor hub assembly mounted to theupper end of the rotor shaft, the inboard blade section of each rotorblade assembly being mounted to the rotor hub assembly such that therotor blade assemblies are rotated by the rotor hub assembly; a bladeactuation shaft concentrically disposed within the main rotor shaft, theblade actuation shaft having an upper end and a lower end; a planetarygear drive located within the rotor hub and engaging the upper end ofthe blade actuation shaft with the output pinion; a drive gear on thelower end of the main rotor shaft and having a first set of teeththereon; a blade actuation drive gear on the lower end of the bladeactuation drive shaft and having a first set of teeth thereon; a primarydrive unit for transmitting rotary motion from the main rotor shaft tothe blade actuation shaft, the primary drive unit includingan input gearengaged with the first set of teeth on the drive gear, a primary drive,a master clutch disposed between the input gear and the primary drivefor controlling transmission of rotary motion therebetween, an outputgear engaged with the first set of teeth on a blade actuation drivegear, an output drive engaged with the output gear, a forward planetarygearset operative for transmitting rotary motion from the primary driveto the output drive, a forward clutch for engaging the forward planetarygearset with the primary drive and the output drive, the engagement ofthe forward planetary gearset causing the blade actuation shaft torotate in the same direction as the rotation of the main rotor shaft, areverse planetary gearset operative for transmitting rotary motion fromthe primary drive to the output drive, and a reverse clutch for engagingthe reverse planetary gearset with the primary drive and the outputdrive, the engagement of the reverse planetary gearset causing the bladeactuation shaft to rotate in the opposite direction from the rotation ofthe main rotor shaft; and a rotor blade actuation control unit forcontrolling engagement and disengagement of the forward and reverseclutches.
 11. A variable diameter rotor system according to claim 10wherein the rotor blade actuation control unit includes a profile forcontrolling the engagement and disengagement of at least one of theclutches.
 12. A variable diameter rotor system according to claim 11wherein the profile includes ramp-up and ramp-down portions during whichtimes the at least one clutch is permitted to slip.
 13. A variablediameter rotor system according to claim 11 wherein the primary driveunit further includes a brake which is adapted to engage with theprimary drive, and wherein the rotor blade actuation control unit usesthe profile to control engagement and disengagement of the brake.
 14. Avariable diameter rotor system according to claim 13 wherein the rotorblade actuation control unit utilizes the profile to modulatedisengagement of the brake while concomitantly modulating engagement ofthe master clutch.
 15. A variable diameter rotor system according toclaim 10 wherein the gear ratio between the drive gear and the inputgear is about 1:2.
 16. A variable diameter rotor system according toclaim 10 wherein the gear ratio between the output gear and the bladeactuation drive gear is about 2:1.
 17. A variable diameter rotor systemaccording to claim 10 further comprising a brake unit adapted to engagethe main rotor shaft and the blade actuation shaft for locking the bladeactuation shaft to the main rotor shaft.
 18. A variable diameter rotorsystem according to claim 10 further comprising a secondary drive unitfor transmitting rotary motion from the main rotor shaft to the bladeactuation shaft in the event of failure of the primary drive unit, thesecondary drive unit includinga secondary input gear engaged with asecond set of teeth on the drive gear, a secondary output gear engagedwith a second set of teeth on a blade actuation drive gear, a secondarydrive, an secondary output drive engaged with the secondary output gear,a secondary master clutch disposed between the secondary input gear andthe secondary drive for controlling transmission of rotary motiontherebetween, a secondary forward planetary gearset operative fortransmitting rotary motion from the secondary drive to the secondaryoutput drive, a secondary forward clutch for engaging the secondaryforward planetary gearset with the secondary drive and the secondaryoutput drive, the engagement of the secondary forward planetary gearsetcausing the blade actuation shaft to rotate in the same direction as therotation of the main rotor shaft, a secondary reverse planetary gearsetoperative for transmitting rotary motion from the secondary drive to thesecondary output drive, and a secondary reverse clutch for engaging thesecondary reverse planetary gearset with the secondary drive and thesecondary output drive, the engagement of the secondary reverseplanetary gearset causing the blade actuation shaft to rotate in theopposite direction from the rotation of the main rotor shaft; andwhereinthe rotor blade control unit controls actuation of the clutches in thesecondary drive unit.